Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 791: What protein does LacY in the Lac Operon code for?

A. Galactoside Permease (Correct Answer)
B. Thiogalactoside Transacetylase
C. Repressor
D. β-Galactosidase

Explanation: **Correct Answer: Galactoside Permease** - The *lacY* gene in the Lac Operon encodes for **galactoside permease**. - This protein is a **membrane-bound transporter** that facilitates the uptake of lactose into the bacterial cell. *Incorrect: β-Galactosidase* - This enzyme is encoded by the *lacZ* gene, not *lacY*. - Its primary function is to **cleave lactose** into glucose and galactose. *Incorrect: Thiogalactoside Transacetylase* - This enzyme is encoded by the *lacA* gene, not *lacY*. - Its exact physiological role is not fully understood, but it may be involved in the **detoxification of non-metabolizable thiogalactosides**. *Incorrect: Repressor* - The Lac Repressor protein is encoded by the *lacI* gene, which is located upstream of the Lac Operon genes (*lacZ, lacY, lacA*). - Its function is to **bind to the operator region** and inhibit transcription of the operon in the absence of lactose.

Question 792: Which of the following transcription factors is MOST essential for maintaining the pluripotency of embryonic stem cells?

A. Nanog, another transcription factor
B. GJA1
C. Oct-4, a transcription factor (Correct Answer)
D. Sox2, another transcription factor

Explanation: ***Oct-4, a transcription factor*** - **Oct-4** (POU5F1) is part of the **core pluripotency network** along with Sox2 and Nanog, essential for maintaining embryonic stem cell self-renewal and undifferentiated state - Among the core factors, **Oct-4 is often considered the most indispensable** as its knockout results in immediate loss of pluripotency and differentiation into trophectoderm - Oct-4 is required for **iPSC reprogramming** (Yamanaka factors) and its expression levels critically determine cell fate decisions - While all three core factors work synergistically, **Oct-4's hierarchical importance** in the regulatory network makes it the most essential single factor *Nanog, another transcription factor* - **Nanog** is a crucial member of the core pluripotency triumvirate (Oct-4, Sox2, Nanog) - Works synergistically with Oct-4 and Sox2 to maintain the undifferentiated state - While essential for pluripotency, **Nanog-null embryos can progress further in development** than Oct-4 knockouts, suggesting Oct-4 has greater fundamental importance - Nanog expression is regulated by and dependent on Oct-4-Sox2 heterodimers *GJA1* - **GJA1** (connexin 43) encodes a **gap junction protein** involved in intercellular communication - **Not a transcription factor** and plays no direct role in regulating pluripotency gene networks - While gap junctions may facilitate signaling in stem cell niches, GJA1 is not part of the core pluripotency machinery *Sox2, another transcription factor* - **Sox2** is the third member of the core pluripotency network and forms critical heterodimers with Oct-4 - Essential for maintaining pluripotency and activating pluripotency-associated genes - While vital, **Sox2 knockout embryos can survive longer** than Oct-4 knockouts, and Sox2's function is largely dependent on Oct-4 partnership - Required for iPSC generation but works primarily through cooperation with Oct-4

Question 793: What term describes the observable traits of an organism resulting from gene expression?

A. Genotype
B. Phenotype (Correct Answer)
C. Genome
D. Morphology

Explanation: **Phenotype** is the correct answer. ***Correct: Phenotype*** - Refers to the **observable characteristics** of an organism resulting from gene expression - Includes physical appearance, biochemical properties, physiological traits, and behavior - Results from the interaction between **genotype and environment** - Represents the **expressed traits** that can be directly observed or measured *Incorrect: Genotype* - Refers to the **genetic makeup** (DNA sequence/alleles) of an organism - Represents the **inherited instructions**, not the expressed traits - Cannot be directly observed without genetic testing *Incorrect: Genome* - The **complete set of genetic material** (all DNA) in an organism - Includes both coding and non-coding sequences - Represents the entire genetic blueprint, not specifically expressed traits *Incorrect: Morphology* - Refers to the **study of form and structure** of organisms - A descriptive term or field of study, not the term for expressed traits - More limited in scope than phenotype (which includes functional traits)

Question 794: Gene for insulin responsive glucose transporter is located on chromosome?

A. 7
B. 21
C. 17 (Correct Answer)
D. 13

Explanation: ***17*** - The gene for the **insulin-responsive glucose transporter**, specifically **GLUT4**, is located on **chromosome 17**. - GLUT4 plays a crucial role in glucose uptake in **muscle** and **adipose tissue** in response to insulin signaling. *7* - Chromosome 7 contains genes for various other important proteins, but not the primary insulin-responsive glucose transporter. - For example, the **cystic fibrosis transmembrane conductance regulator (CFTR)** gene is located on chromosome 7. *21* - **Chromosome 21** is well-known for its association with **Down syndrome**, a condition caused by trisomy 21. - It does not contain the gene for the insulin-responsive glucose transporter. *13* - Chromosome 13 is associated with several genetic disorders, including **retinoblastoma** (due to a mutation in the *RB1* gene). - It is not the location of the gene for the insulin-responsive glucose transporter.

Question 795: What is the role of catabolite activator protein (CAP) in lac operon?

A. Attenuation
B. Constitutive expression
C. Positive regulator (Correct Answer)
D. Negative regulator

Explanation: ***Positive regulator*** - **Catabolite activator protein (CAP)**, also known as **cAMP receptor protein (CRP)**, binds to **cAMP** when glucose levels are low. - This complex then binds to the **promoter region** of the lac operon, enhancing the binding of **RNA polymerase** and thereby increasing **transcription** (positive regulation). *Negative regulator* - A negative regulator would **inhibit gene expression**, such as the **lac repressor** which binds to the operator in the absence of lactose. - CAP, in contrast, promotes gene expression when **glucose is absent** and **lactose is present**. *Attenuation* - **Attenuation** is a regulatory mechanism primarily observed in operons involved in amino acid synthesis, such as the **trp operon**. - It involves the premature termination of transcription due to the formation of specific **mRNA secondary structures**, which is not the primary role of CAP. *Constitutive expression* - **Constitutive expression** refers to the constant expression of genes regardless of environmental conditions. - The lac operon is a **regulated operon**, meaning its expression is turned on or off in response to cellular needs, not expressed constitutively.

Question 796: At which cell cycle checkpoint is the cell cycle halted if the cell's DNA is damaged?

A. G1 - S
B. G2 - M (Correct Answer)
C. S - G2
D. G0 - G1

Explanation: ***G2 - M*** - The **G2/M checkpoint** is the **primary checkpoint for DNA damage detection** in the cell cycle. - This checkpoint ensures that **all DNA is properly replicated and any DNA damage is repaired** before the cell enters mitosis. - If **DNA damage** is detected at the G2/M checkpoint, the cell cycle is halted through **p53-mediated pathways**, preventing damaged DNA from being passed to daughter cells. - This is the **most critical safeguard** against transmission of genetic errors. *G1 - S* - The **G1/S checkpoint** (restriction point) primarily monitors cell size, nutrient availability, and growth signals to determine if the cell should commit to DNA replication. - While it does perform **some DNA damage surveillance**, it is not the primary DNA damage checkpoint. - Its main function is ensuring the cell is ready to **initiate** DNA synthesis, not specifically to detect DNA damage. *S - G2* - The **intra-S checkpoint** monitors DNA replication during S phase and can detect replication errors. - However, this represents a monitoring process during synthesis rather than a major decision checkpoint for halting the cycle due to DNA damage. - The transition from S to G2 itself is not a recognized major checkpoint. *G0 - G1* - The **G0 phase** is a quiescent state where cells exit the active cell cycle. - The G0-G1 transition involves cells re-entering the cell cycle in response to growth signals, not a DNA damage checkpoint. - This is not a recognized checkpoint for DNA damage assessment.

Question 797: What is the chromosomal location of the ABO gene?

A. 9q34.2 (Correct Answer)
B. 10p15.3
C. 11q23.3
D. 12p12.1

Explanation: **Correct: 9q34.2** - The **ABO gene** is located on the **long arm (q) of chromosome 9 at position 34.2 (9q34.2)** - This gene encodes glycosyltransferases that determine **A, B, AB, and O blood groups** by adding specific sugar residues to H antigen - Understanding this chromosomal location is important for **genetic counseling**, **paternity testing**, and **inheritance pattern analysis** - The ABO blood group follows **Mendelian codominant inheritance** with the O allele being recessive *Incorrect: 10p15.3* - This location on chromosome 10 short arm is not associated with the ABO gene - This region contains genes associated with conditions like **CHARGE syndrome** *Incorrect: 11q23.3* - This region on chromosome 11 contains the **MLL (KMT2A) gene** - Associated with various **hematological malignancies** including acute leukemias - Not the location of blood group antigen genes *Incorrect: 12p12.1* - This chromosome 12 location contains genes involved in other metabolic pathways - Associated with genes like **COL2A1** (collagen type II) and others - Not related to ABO blood group determination

Question 798: Which of the following statements regarding cell division is NOT applicable to all types of cell division?

A. Results in genetically identical daughter cells
B. Produces two daughter cells (Correct Answer)
C. Maintains the same chromosome number as the parent cell
D. Produces a haploid number of chromosomes only in meiosis

Explanation: ***Produces two daughter cells*** - This is the **most fundamental distinction** between mitosis and meiosis that is NOT universal to all cell division. - **Mitosis** produces exactly **two diploid daughter cells** from one parent cell. - **Meiosis** produces **four haploid daughter cells** after two sequential divisions (Meiosis I and Meiosis II). - Since meiosis produces four cells, not two, this statement is **NOT applicable to all types of cell division**. *Results in genetically identical daughter cells* - This statement is true for **mitosis**, where daughter cells are clones of the parent cell. - However, **meiosis** produces genetically diverse cells due to crossing over and independent assortment. - While also not universal, this is a secondary characteristic difference. *Maintains the same chromosome number as the parent cell* - This is true for **mitosis** (diploid → diploid, 2n → 2n). - **Meiosis** reduces chromosome number by half (diploid → haploid, 2n → n). - This is another important but secondary distinguishing feature. *Produces a haploid number of chromosomes only in meiosis* - This statement is **universally true** and accurately describes a defining characteristic of meiosis. - It IS applicable as a correct description of cell division types. - Unlike the other options, this statement doesn't claim something false about any cell division type - it simply states a fact about meiosis.

Question 799: Most commonly used vector for DNA cloning?

A. Plasmid (Correct Answer)
B. Virus
C. Cosmid
D. Phage

Explanation: ***Plasmid*** - **Plasmids** are the most commonly used vectors for DNA cloning due to their small size, ease of manipulation, and ability to replicate independently within a host cell. - They contain essential features like an **origin of replication**, **multiple cloning sites**, and **antibiotic resistance genes**, facilitating the insertion, selection, and amplification of foreign DNA. *Virus* - While viruses can be used as vectors (e.g., **adenoviruses** and **retroviruses**), they are typically more complex to handle and are often reserved for specialized applications like gene therapy, not routine DNA cloning. - They also have **safety concerns** and can elicit immune responses in the host, making them less suitable for general cloning compared to plasmids. *Cosmid* - **Cosmids** are hybrid vectors that combine features of plasmids and bacteriophages, designed to carry larger DNA inserts (up to 45 kb). - While useful for cloning large DNA fragments, their larger size and more complex structure make them less versatile and less commonly used for everyday cloning compared to the smaller, more manageable plasmids. *Phage* - **Bacteriophages** (phages) like lambda phage are specifically designed to infect bacteria and can accommodate large DNA inserts (up to 20 kb). - Phage vectors are often employed for constructing **genomic libraries** due to their capacity for large inserts, but their application is more specialized than the broad utility of plasmids in basic DNA cloning.

Question 800: Which of the following statements about chimeric DNA is false?

A. Sticky ends produced by restriction endonucleases facilitate the formation of chimeric DNA.
B. Chimeric DNA does not require DNA ligases. (Correct Answer)
C. The organism harboring chimeric DNA exhibits features of both itself and the properties of the insert.
D. Chimeric DNA is formed by linking DNA fragments of unrelated genomes.

Explanation: ***Chimeric DNA does not require DNA ligases.*** - This statement is **false** because **DNA ligases** are essential enzymes that catalyze the formation of a **phosphodiester bond** between the 3'-hydroxyl and 5'-phosphate ends of DNA fragments, sealing the nicks to create a stable recombinant DNA molecule. - Without DNA ligase, the inserted DNA fragment would only be held by hydrogen bonds between complementary sticky ends, making the **chimeric DNA molecule unstable** and prone to dissociation. *Sticky ends produced by restriction endonucleases facilitate the formation of chimeric DNA.* - **Restriction endonucleases** cleave DNA at specific recognition sites, often creating **sticky ends** (single-stranded overhangs) that are complementary to each other. - These sticky ends allow for specific and efficient **annealing** of DNA fragments from different sources, which is a crucial step in the formation of chimeric DNA. *The organism harboring chimeric DNA exhibits features of both itself and the properties of the insert.* - When chimeric DNA is **introduced into an organism** (e.g., through transformation), the inserted gene can be expressed, leading to the production of new proteins. - This expression can confer **new traits or functions** to the host organism, combining its original characteristics with those encoded by the foreign DNA. *Chimeric DNA is formed by linking DNA fragments of unrelated genomes.* - **Chimeric DNA**, also known as **recombinant DNA**, is precisely defined as a DNA molecule constructed in vitro by joining DNA segments from **two or more different organisms** or sources. - This process allows for the combination of genetic material that would not naturally occur together, creating novel gene combinations.

Practice by Chapter

DNA Replication and Repair Mechanisms

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Transcription Factors and Gene Regulation

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Epigenetics and DNA Methylation

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RNA Processing and Splicing

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miRNA and RNA Interference

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Protein Synthesis and Post-Translational Modifications

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Genomics and Human Genome Project

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Single Nucleotide Polymorphisms

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Gene Therapy Approaches

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CRISPR-Cas9 and Genome Editing

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DNA Fingerprinting and Forensics

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Molecular Basis of Genetic Diseases

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